This invention relates to a method of transmitting data in a communications network and to a communications network as set forth in the preambles of the independent claims.
Service-on-demand provides for the bidirectional exchange of information between customers and the server network, so that a control channel is needed from the network to the customer and a return channel from the customer to the network. The return channel, generally in combination with a control channel, has several main functions, such as identification of the customer to the network, connection setup and release, interaction with the server, and feeding in of own data to establish a video-telephone call, for example.
An essential feature of the return channel is the bit rate available to the customer.
From an article entitled xe2x80x9cZugangsnetzstrukturen fxc3xcr interaktive Videodienste (Teil 1)xe2x80x9d, published in xe2x80x9cDer Fernmeldeingenieurxe2x80x9d, No. 8, 1994, it is known that different transmission techniques are used for the return channel. Frequency-division multiplexing, time-division multiplexing or code-division multiplexing techniques are used to transmit data from the different network terminations in the return channel to the head end. Central or decentralized control is provided for the different transmission techniques to avoid collisions between the data of different network terminations. Via a control channel, the data terminal is granted permission to transmit or the transmission is interrupted. The assignment of time slots in time-division multiplexing, for example, can be dynamically adapted to the customer""s requirements. For this, however, return channels with fixed data rates are used. For the transmission of high data rates, the return channels are generally band-limited and constitute bottlenecks. In addition, the return channel is affected by noise. To solve these problems, the number of bits per symbol of the carrier must be increased, which, however, increases the sensitivity to noise phenomena. To suppress the noise, the number of bits per symbol would have to be reduced.
One approach to solve these problems is to use OFDM (orthogonal frequency division multiplexing) with n-QAM (n-state quadrature amplitude modulation). In this technique, the information to be transmitted is modulated onto a frequency comb, with each frequency of the comb being modulated with a relatively large number of bits per symbol. Using suitable error-correcting mechanisms, transmission errors and noise-induced interference can thus be minimized. The solution using OFDM requires a relatively complex modulator in the terminal, which adds to the cost of the terminal.
The method according to the invention has the advantage that the data rate in the return channel can be adapted to the transmission quality of the return channel by setting suitable modulation methods in the network terminations. This makes it possible in a simple manner to establish an adaptive system in which
The data rate can be made dependent on the quality of the return channel. For a very poor connection, this means that transmission takes place at a low data rate in order to avoid large transmission errors. The data rate is determined by the type of modulation of the carrier in the return channel. For a poor return channel, a robust modulation method, such as QPSK (quadrature phase-shift keying), will be chosen, while in case of good transmission quality, 16 QAM, for example, can be used.
By the measures recited in the subclaims, the method set forth in the independent claim can be improved.
Particularly advantageously, the transmission qualities of the return channels are determined centrally at the head end, which calculates therefrom a criterion on the basis of which a control signal sets the modulation methods in the network terminations.
Advantageously, modulation schemes are used which produce different numbers of bits per symbol, thus rendering the transmission channel robust or less robust.
Advantageously, to evaluate the quality of the channel, the signal-to-noise ratio of the signal is evaluated.
According to another advantageous aspect of the invention, the method can be used both in a wirelines network and in a radio network.